1. Field of the Invention
This invention relates to a synchronizing signal generator for deriving synchronizing signals in dependence on a video signal, and more particularly to a synchronizing signal generator suitable for use in a television transmitter of a television broadcast station or a domestic television receiver.
2. Description of the Prior Art
FIGS. 1A and 1B show block diagrams of two known synchronizing signal generators for television transmitters.
In the synchronizing signal generator of FIG. 1A, an externally supplied input video signal VDS is applied to a burst separator circuit 1 and to a horizontal and vertical synchronizing separator circuit 2. A horizontal synchronizing signal and a vertical synchronizing signal are separated from the video signal by the synchronizing separator circuit 2. A color burst signal is separated from the video signal by the burst separator circuit 1, to which the horizontal and vertical synchronizing signals are applied to obtain burst gate pulses for separating the burst signal.
The separated burst signal is applied to a phase discriminator 3. An output of a quarter frequency divider 4, which forms a comparison signal, is compared in phase with the burst signal by the phase discriminator 3. An output voltage is obtained from the phase discriminator 3 in dependence on the phase difference between the burst signal and the comparison signal. The output voltage of the phase discriminator 3 is applied to a voltage-controlled oscillator 5, as a control signal. The oscillation frequency of the oscillator 5 is controlled by the output voltage of the phase discriminator 3. The controlled oscillation frequency (14.3 MHz) is four times the frequency (3.58 MHz) of the burst signal.
The output frequency of the oscillator 5 is divided by the quarter frequency divider 4, and is applied to the phase descriminator 3 to be compared with the burst signal. The output frequency of the oscillator 5 is also applied to a frequency divider 6, as a standard frequency signal, and it is divided by the frequency divider 6. Synchronizing signals synchronized with the synchronizing signals contained in the input video signal are obtained from the frequency divider 6. This is sometimes referred to as gen-lock.
A color burst signal (3.58 MHz) is obtained by 1/4 division of the standard frequency signal (14.3 MHz); a horizontal synchronizing signal (15.73 KHz) is obtained by 1/2.multidot. (1/455) division of the standard frequency signal; and a vertical synchronizing signal (59.9 Hz) is obtained by (1/525) .multidot. (1/455) division of the standard frequency signal, by the frequency divider 6. The horizontal and vertical synchronizing signals are synchronized with corresponding signals in the video signal, by supplying outputs of the synchronizing separator circuit 2, namely horizontal and vertical synchronizing signals, as reset pulses to the frequency divider 6.
In the other synchronizing signal generator, shown in FIG. 1B, parts which correspond to parts in FIG. 1A are denoted by the same reference numerals. The output signal P of the phase discriminator 3 is applied as a control signal to the voltage-controlled oscillator 5 through a resistor 7, and it is further applied to an input terminal of a low frequency amplifier 35. An output voltage q of the low frequency amplifier 35 is applied to the oscillator 5.
The low frequency amplifier 35 comprises resistors 8 and 9, capacitors 10 and 11, and an amplifier 12. The output signal P of the phase discriminator 3 is applied to the amplifier 12 through an integrating circuit comprising the resistor 8 and the capacitor 10. The non-inverting amplification factor of the amplifier 12 is about a thousand. The output of the amplifier 12 is smoothed by the capacitor 11, and applied through the resistor 9 to the oscillator 5, as the signal q. The signal q is a low frequency signal which is nearly a dc voltage, due to the function of the integrating circuit and the capacitor 11. The levels of the signals P and q vary with deviations .DELTA.f from the central frequency at the phase discriminator 3, as shown in FIG. 2.
In the automatic frequency control loop including the phase discriminator 3, the quarter frequency divider 4 and the oscillator 5, the control signal P functions as a lock-in signal quickly to control frequencies which deviate substantially from the central frequency, and the control signal q functions as a lock-hold signal to control frequencies which deviate only slightly from the central frequency.
In the known circuits shown in FIGS. 1A and 1B, the output voltage of the phase discriminator 3 is applied to the voltage-controlled oscillator 5, as the control signal. The oscillator 5 is designed to continue oscillating when the synchronizing signals in the input video signal include noise or disappear. However, when a normal input video signal is again received, there is the possibility that there is a difference between the self-oscillating frequency of the oscillator 5 and the frequency to be synchronized. In that case, the frequency of the oscillator 5 varies until it is synchronized with the frequency of the synchronizing signal in the input video signal, that is until gen-lock is again established. In this case, the frequencies of the signals obtained from the frequency divider 6 vary, and so the picture is distorted.